Short Notes K127

phys. stat. sol. (a) 56, Kl27 (1979) Subject classification: 1.2; 22.6 Department of Physics, University of Helsinki') (a) and Institute of Physics, Academy of Sciences of the Estonian SSR, Riga (b) High Pressure Transformations in Cuprous Oxide

BY M. KALLIOMAKI (a), V. MEISALO (a), and A. LAISAAR (b)

-

The rather open structure of cuprous oxide Cu20 (cuprite) suggests the possi- bility of a crystallographic phase transformation to a denser packing arrange- ment under high pressure. There a re also several other physical observations indicative of phase instability. For example, both the thermal expansion co- efficient cc and the Griineisen constant t assume negative values in certain regions of the p-T plane /1 to 4/. In a number of other crystals this generally implies an impending phase transformation. Furthermore, the elastic proper- ties of Cu20 exhibit anomalous behaviour under pressure /5, 6/, with a Poisson ratio a s high a s 0.445. On the basis of their data up to 0.3 GPa, Manghnaniet al. /6/predicted a high pressure transformation. Popova et al.

/7/ studied the high pressure behaviour of Cu20, while Belash et al. /8/ cor- rected and extended the former results reporting curves of both disintegration and synthesis. In both these investigations the material was studied at ambient pressure after quenching to room temperature. Since there was no apparent evidence of a crystallographic phase transformation, any possible transformation within the studied pressure and temperature range would have to be reversible and not too sluggish. Belash et al. predicted one at 13 GPa, room temperature. Young et al. /9/ studied the Seebeck coefficient of Cu20 up to 6 GPa, and their resistance curves extend to 9 GPa. In their data there is no indication of a phase transformation, but the sensitivity of their measurements was rather low. The other available high pressure data /10 to 12/ extend only to about 1.2 GPa. In the absence of any direct evidence of the expected phase trans- formation, we wanted to perform an exploratory study on the high pressure behaviour of Cu20 in situ.

1) Siltavuorenpenger 20 D, SF-001 70 Helsinki 17, Finland.

K128 physica status solidi (a) 56

T a b l e 1

X-ray diffraction lines and indexing to a cubic pseudocell for Cu20 I1 at 5.0 GPa

2.978 2.982 2.427 2.435 2.120 2.109 1.486 1.491 1.274 1.271 a = 4.217%

200 22 0 31 1

Several high pressure samples of Cu20 were prepared from small crystals of optical quality and studied in a diamond anvil cell under different hydrostatic o r quasi-hydrostatic pressure conditions. For optical observations a laboratory microscope with a simple monochromator was utilised, and X-ray powder diffraction studies were performed using a semi-microfocus generator ( M o s ) and photographic recording. The pressures were calibrated against the NaCl scale. The highest applied pressures were of the order of 15 GPa. The optical studies were extended to 550 K, while X-ray data were obtained at room tem- perature only. Optical studies showed that there is a change of colour due to a discontinuous jump to longer wavelengths in the position of the absorption edge at 5.0 GPa. In powder samples a weak Becke line can be observed under a pressure gradient. Above 5 GPa the X-ray patterns have weak, broad in- tensity maxima in contrast with those obtained at somewhat lower pressures. These are qualitative indications of a phase transformation. A l l stronger X-ray lines of the original pattern can be found also above this pressure, but with small consistent changes in relative positions. (Some of the broad lines a re probably to be regarded as doublets o r triplets.) Neither the changes in optical absorption nor in the X-ray data could be due to the disintegration of Cu20. The birefringency of the new phase, denoted by Cu20 11, is weak. This supports our qualitative conclusion reached on the basis of the data of Table 1, that it is assignable to a distortion of the original cubic structure. We note that any non-hydrostatic strains in the samples apparently have large effects on the behaviour of Cu20 in this transformation.

Short Notes

T a b l e 2

X-ray data for Cu20 111 at 13 GPa

K129

2.421 2.218 2.121

4 2.020 5 1.922 6 1.700 7 1.617 i 8 1.455

vw vw

~ vw

At 12 GPa cuprous oxide becomes opaque to visible light. This phenomenon is reversible, but at pressures of the order of 15 GPa irreversible chemical disintegration is observed. The few X-ray diffraction lines recorded at 13 GPa are presented in Table 2. It appears that the new phase, Cu20 111, must have a structure essentially different from the original one.

It was shown by high pressure microscopy that at elevated temperatures the transformation Cu20 I-11 shifts to higher pressures, a s indicated in Fig. 1. The transformation 11-111, on the other hand, shifts steeply towards lower pressures with increasing temperature and follows closely the disintegration line reported by Belash et al. /8/. We locate the triple point at 430 K, 7.0 GPa, close to the kink in the disintegration curve. (Compare our phase diagram with that of AgaO /13/.)

The transformation Cu20 I-11 wp find to be close to the theoretical disinte- gratioh Jrne of Belash et al. /8/. We expect that both the volume discontinuity and the change in the Gibbs free energy G must be much smaller for

Fig. 1. The phase diagram of Cu 0 according to our optical data (solid lines) and fmes of disinte- gration and synthesis (broken lines, cf. / 8 / ) . The triangles a re the transformation points found from our X-ray data and the solid circle denotes the observed disintegration at room temperature

K130

this crystallographic transformation than for disintegration. The small volume

discontinuity observed by the Becke line technique is also consistent with our

assumption of the distortive nature of this transformation. The 11-III trans- formation is evidently more radical, presumably with a larger volume dis-

continuity and a reorganisation in the .packing of atoms. It was shown to be

possible to perform reversible excursions into the stability range of phase III at all studied temperatures; the disintegration is a property of this phase only. Altogether, our data confirm the results of Belash et al. and a r e in close

agreement with their prediction of a high pressure transformation at 13 GPa.

physica status solidi (a) 56

Finally, w e note that the transformation to phase I11 in Cu20 appears to

be accompanied by a major change in electronic and binding properties of Cu.

The optical properties of Cu20 111 give qualitative support to the idea of a Mott

transition to a metallic phase at high pressures for this type of materials. It would be of great interest to obtain reliable information on the electrical prop-

er t ies of Cu20 under the relevant pressure and temperature conditions.

Leningrad State University, supplying us with the original Cu20 crystals, and

to The Finnish-Soviet Commission for Scientific and Technical Cooperation

for the travel grants which made this work possible.

We are indebted to F. Kreingold and K. Lider, Institute of Physics,

References

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/4/ G.K. WHITE, J. Phys. C - 11, 2171 (1978).

/5/ J. HALLBERG and R.C. HANSON, phys. stat. sol. - 42, 305 (1970). /6/ M.H. MANGHNANI, W.S. BROWER,and H.S. PARKER, phys. stat. sol.

/7/ S. V. POPOVA, N. R. SEREBRYANAYA,

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Short Notes K131

/11/ A.I . NIILISK, A.V. GERST, and YA.YA. KIM, Fiz. tverd. Tela - 7, 931

/12/ A. LAISAAR, A. NIILISK, F. KREINGOLD, and K. LIDER, Proc. VII. Internat. AIRAPT Conf. High Pressures. Le Creusot (France) 10 .7 . to 3.8.1979 (paper H VII 1).

(1965).

/13/ W. KLEMENT, Jr . , phys. stat. sol. (a) - 39, K45 (1977).

(Received September 24, 1979)